Monoclonal antibody 33.1, prepared against a human EBV-transformed B cell line has been characterized in detail. The 33.1 antigen is undetectable on normal peripheral blood mononuclear cells but is present on most EBV-transformed B cell lines and B-CLL cells, on some other B cell-derived neoplastic cells and on normal B cells activated with EBV, pokeweed mitogen and dexamethasone. The antigen is a two-chain molecular with clear homology to human HLA-DR antigens but it is distinct from DR in structural features and in cellular distribution. Rabbit-mouse Hybridoma H105-14 has been shown to secrete a 19S IgM with H and L chains of rabbit origin. The source of the presumed J chain (rabbit or mouse) is under investigation. Hybridoma H89, which expresses no group a allotype but appears to secrete an a 2/H chain by sequence is being analyzed. Human-human T cell hybridomas prepared in collaborative experiments from cells of infectious mononucleosis patients have been shown to have nonspecific antibody-suppressing activity.